Evaluation of Guillain-Barré Syndrome among recipients of influenza vaccine in 2000 and 2001.

BACKGROUND: The 1976-1977 swine influenza vaccine was associated with an elevated risk of Guillain-Barré Syndrome (GBS), especially within 6 weeks after vaccination. A 2004 IOM report concluded that evidence was inadequate to accept or reject a causal relationship between subsequent influenza vaccine formulations and GBS. Studies published after the IOM report have been limited by passively reported data or lack of validation of coded diagnoses. PURPOSE: To evaluate whether influenza vaccine is associated with GBS. METHODS: Controlled observational study using national data from the Medicare program, which ensures a predominantly elderly population. People included had a Medicare claim for influenza vaccination during September-December in 2000 or 2001. Medical records were reviewed to classify definite, probable, or possible GBS (or not a case) using a standardized case definition. In a risk interval design, the incidence rate of GBS during Weeks 0-6 after vaccination (exposed period) was compared to Weeks 9-14 after vaccination (comparison period). Data collection occurred during 2003-2007, and analysis was conducted during 2007-2009. RESULTS: Primary analysis included 22.2 million vaccinees, among whom 164 definite or probable GBS cases with onset during Weeks 0-6 or 9-14 were identified. The incidence rate ratio (IRR [95% CIs]) based on the GBS rate in the vaccine-exposed versus comparison periods, was 1.04 (0.76, 1.43) for combined years; 0.86 (0.52, 1.41) among people vaccinated in 2000; and 1.21 (0.79, 1.86) among people vaccinated in 2001. Secondary analysis additionally included 74 possible GBS cases; results were similar. CONCLUSIONS: Overall, the results do not support an association between influenza vaccine receipt and GBS among the elderly for the years studied (2000-2001 and 2001-2002 formulations).

Evaluating adverse events after vaccination in the Medicare population.

PURPOSE: Post-licensure observational studies using large linked databases can provide important data about whether adverse events are associated with vaccines, but databases that have been used may not have sufficient statistical power to examine rare events, and may underrepresent the elderly. We assessed the utility of Medicare data for evaluating adverse events after influenza and pneumococcal vaccines, by using an example involving selected clinical conditions, and evaluating aspects of data quality relevant to vaccine safety analyses. METHODS: We used 2001 data from the National Claims History File and Enrollment Database to determine if hospitalization for urinary tract infection (not likely associated with vaccination) or for cellulitis and abscess of the upper arm and forearm is associated with vaccination. RESULTS: For influenza vaccine, the 7-day period after vaccination did not demonstrate an elevation in hospitalization with cellulitis and abscess of the upper arm and forearm; for pneumococcal vaccine, a clear peak was evident. No increase in urinary tract infection was found after either influenza or pneumococcal vaccine. Having a prior Medicare claim for pneumococcal vaccine within 5 years was a risk factor for hospitalization with cellulitis and abscess of the upper arm and forearm (relative risk, 2.6; 95% confidence limits (CL), 1.3, 5.0). CONCLUSIONS: Medicare data are a useful source for evaluating adverse events after vaccination. Screening analyses can be performed using administrative data, but medical record review to validate diagnoses will often be needed for rigorous study of vaccine-adverse event associations.